http: document master's keep-alive related option http11-socket

See unbit/uwsgi#1097

HTTP.rst

@@ -93,6 +93,14 @@ If your backends set the correct HTTP headers, you can use the
 Also remember to set "Connection: Keep-Alive" in your response. You can
 automate that using the ``add-header "Connection: Keep-Alive"`` option.
 
+Since uWSGI 2.1 (master branch) you can use the ``http11-socket`` option.
+``http11-socket`` may replace the ``add-header`` and ``http-keepalive`` options
+(but it doesn't touch tcp stuff as so ``so-keepalive`` does).
+Once set the server will try to maintain the connection opened if a bunch of
+rules are respected. This is not a smart http 1.1 parser (to avoid parsing the
+whole response) but assumes the developer is generating the right headers.
+``http11-socket`` has been added to support RTSP protocol for video streaming.
+
 HTTP auto gzip
 -------------